Exploring, drilling and completing hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. In recognition of these expenses, added emphasis has been placed on efficiencies associated with well completions and maintenance over the life of the well. Over the years, ever increasing well depths and sophisticated architecture have made reductions in time and effort spent in completions and maintenance operations of even greater focus.
Completions and maintenance operations often involve the utilization of isolation mechanisms such as packers, plugs, and other downhole devices. Such devices may be used to sealably isolate one downhole section of the well from another as an application is run in one of the sections. Indeed, a considerable amount of time and effort may be spent achieving such isolations in advance of running the application, as well as in removing the isolation mechanism following the application. For example, isolations for perforating and fracturing applications may involve a significant amount of time and effort, particularly as increases in well depths and sophisticated architecture are encountered. These applications involve the positioning of an isolation mechanism in the form of a plug. More specifically, a bridge plug may be located downhole of a well section to be perforated and fractured. Positioning of the bridge plug may be aided by pumping a driving fluid through the well. This may be particularly helpful where the plug is being advanced through a horizontal section of the well.
Once in place, equipment at the oilfield surface may communicate with the plug over conventional wireline so as to direct setting thereof. In the circumstance of a cased well, such setting may include expanding slips of the plug for a biting interfacing with a casing wall of the well and thereby anchoring of the plug in place. A seal of the plug may also be expanded into sealing engagement with the casing. This may be achieved by way of the seal element swelling or by way of compression on the seal during setting that forces the seal into radial expansion and engagement with the casing. Regardless, both anchored structural security and sealed off hydraulic isolation may be achieved by the plug once it is set.
Once anchored and hydraulically isolated, a perforation application may take place above the plug so as to provide perforations through the casing in the corresponding well section. Similarly, a fracturing application directing fracture fluid through the casing perforations and into the adjacent formation may follow. This process may be repeated, generally starting from the terminal end of the well and moving uphole section by section, until the casing and formation have been configured and treated as desired.
The presence of the set bridge plug as indicated above keeps the high pressure perforating and fracturing applications from affecting well sections below the plug. Indeed, even though the noted applications are likely to generate well over 5,000-10,000 PSI, the well section below the plug is kept isolated from the section thereabove. This degree of secure isolation is achieved due to the durable slips and central mandrel in combination with a reliable seal element as described above.
Unfortunately, unlike setting of the bridge plugs, wireline communication is unavailable for releasing the plugs. Rather, due to the high pressure nature of the applications and the degree of anchoring and sealing required of the plugs, they are generally configured for near permanent placement once set. As a result, removal of the bridge plugs may require a challenging milling or drill-out interventional application.
In recognition of the challenges to plug removal, the types of materials and construction of such isolation mechanisms has changed. For example, cast iron plug construction has given way to aluminum plug construction which is much easier to drill out by way of a conventional coiled tubing application. In fact, newer composite plug construction may be used which is even easier to drill out. Specifically, the composite construction of the slips, mandrel and overall framework of a plug may be of a specific gravity that is well under 2.0, absorb water and/or be degradable by design.
Unfortunately, material choices for the seal element of the plug may not be selected based primarily on ease of subsequent drill out applications. That is, unlike the other framework of the plug, the seal element is intentionally configured with substantial elongation to break properties (e.g. elongation properties), perhaps 200%-400% or more. This allows the seal element to compressibly attain an effective hydraulic isolation as detailed above. However, it presents a significant challenge to effective drill-out of this portion of the plug. Thus, removal of a series of plugs following stimulation may take considerable time.
As a practical matter, an even larger issue is presented by the substantial elongation properties of the seal element. Namely, it is likely that rather than just degrading into fine particles during drill out, the seal element will often stretch and tear off into larger chunks. This may result in clogging of lines at the oilfield surface as the materials are flowed back to surface. Even worse, this debris may not flow back until production, at which time drill out and other cleanout equipment has left the oilfield. Thus, as opposed to tens of thousands of dollars in cleaning out some surface equipment near the time of drill out, the rework may be much more significant. For example, redressing the issue may require hundreds of thousands of dollars in terms of lost time and production spent on shutting down production and re-rigging things for sake of an entirely new cleanout of the well in addition to unclogging lines at surface.